1. Field of the Invention
The present invention relates to an image sensor, and particularly to a high resolution contact type image sensor suppressing interference between adjacent bits.
2. Description of the Prior Art
A contact type image sensor based on an elongated reading element having a sensor section whose width is the same as that of an original document can be used as a large-area image sensing device which does not require a reduction optical system. Such an image sensor includes as its photoelectric converting layer an amorphous semiconductor layer made of a material such as amorphous silicon (a-Si) or a poly-crystalline thin film made of a material such as cadmium sulfide (CdS) or cadmium selenide (CdSe). The image sensor of this type has been widely applied to compact type original document readers, or the like.
As one of the basic structures of the sensor section of this image sensor, there is a sandwich structure. As shown in FIG. 7, a sandwich type sensor element comprises a photoelectric converting layer 104 which is sandwiched between a light permeable upper electrode 103 and a lower electrode 102 formed on a substrate 101. In a contact type image sensor, a plurality of the sandwich type sensor elements are arranged in a row on an elongated substrate (for example, in case of 8 dots per mm, 1728 elements for JIS A-4 and 2048 for JIS B-4).
In order to perform precise reading, these sensor elements must be completely independent from each other and the light receiving region of the sensor elements must have the same area. For this reason, many experiments have been conducted in order to precisely define the light receiving area of each sensor element provided on the elongated substrate.
For example, in each sensor element of the most fundamental contact type image sensor shown in FIG. 8, both the lower electrode 102 and the photoelectric converting layer 104 are separately formed, and further the operating section of the light permeable upper electrode 103 is separately formed so as to define the region of a photoelectric converting layer sandwiched between the lower electrode 102 and the upper electrode 103 as a light receiving area (sensor area). The sensor elements are thus formed separately from each other.
In such a structure, since the lower electrode, the photoelectric converting layer and the upper electrode are formed by means of a photolitho-etching process, the fabricating process is complicated, and further each sensor element tends to have a different light receiving area due to non-alignment of a pattern, or other operational factors. Further, the reliability and the yield rate of the image sensor are degraded because the edge of the photoelectric converting layer disposed along the boundary associated with the fabricating mask is often contaminated and injured during such processes as a photolitho-etching process for forming the upper electrode.
Further, in a contact type image sensor using as its photoelectric converting layer a non-doped amorphous silicon, since an amorphous silicon itself has a high resistance value (electrical resistivity in the dark is approximately 10.sup.9 .OMEGA.cm), the separation (isolation) of adjacent bits (adjacent sensor elements) may be omitted or substantially reduced to zero. For example, as shown in FIG. 9, only the lower electrode 102 is formed separately, while the photoelectric converting layer 104 and the upper electrode 103 are respectively formed in the shape of a belt. In this structure, which is the same as the abovementioned structure, the light receiving area is defined by means of the overlapped region of the upper electrode and the lower electrode. Since a photolitho-etching process is utilized only for the formation of the lower electrode while the upper electrode is selectively formed by means of a sputtering process, or the like, through a metal mask, the fabricating process is simplified. However a photoelectric converting layer region corresponding to the edge of the metal mask is likely to be injured, resulting in the degradation of the yield rate.
In addition, there have been such problems as that of insufficient insulation between adjacent bits causing leakage of signals, and that the light receiving area is larger than the area defined by the lower electrode.
FIG. 10 shows one idea for providing a solution of those problems, in which the light receiving area is defined not by the members constituting the sensor elements, but by means of a light shielding film 105.
An example is shown in FIG. 10, in which the lower electrode 102 is formed by means of a larger pattern, and after a passivation film 107 is coated on the whole surface of a substrate on which the sensor elements have been formed, the light shielding film 105 is formed so as to define the light receiving area.
In this case, the patterning of a window section 100 formed in the light shielding film 105 is made by, for example, a photolitho-etching process in order to achieve a high degree of accuracy. Though these processes are complicated, the resolution is improved to a certain extent.
However, when the higher resolution is desired, space between adjacent bits must be made narrow. However, this results in an increase of the leakage of signals although the resistivity of a dark section of a photoconductive section beneath the light shielding film is high, for example, within the range of 10.sup.9 .OMEGA.cm.
Assuming now that a light receiving region of the sensor element is a square whose side length is 80% of the sensor pitch P, then, the distance between adjacent bits is d.ltoreq.8.3 .mu.m if the density of the sensor element is 24 dot/mm and P=41.6 .mu.m, and the distance d.ltoreq.6.25 .mu.m, if the density of the sensor element is 32 dot/mm and P=31.25 .mu.m. Thus, the distance between adjacent bits is so narrow that a signal leaks between adjacent bits. This prevents resolution of the image sensor from being enhanced.